Create an iMovie project and drag the photos or videos from your desktop into the timeline.
Or, drag the photos or videos directly from the Photos library into the timeline.
Or, use the import button to import the media from your desktop into an iMovie event or project.
Or, click on the photos item in the iMovie project sidebar and drag your photos or videos into iMovie from the media browser. That is the recommended way. For it to work your Photos liibrary need to be designated as your system photo library and the media needs to be contained in the Photos library rather than in the cloud.
-- Rich